The present invention relates to a throwaway insert for a ball end mill for machining e.g. the contour of a mold.
Ordinary throwaway inserts for ball end mills have, as shown in FIG. 5, a substantially oval shape in plan with a flat rake face 1 and thus the cutting edge 5 is at the same level over the whole length.
The insert shown in FIG. 6 is a modified version, in which rake faces 1a, 1b near each nose 2 are angled to each other to vary the height of the cutting edge for smoother contact with a workpiece.
Further, a throwaway insert disclosed in unexamined Japanese utility model publication 1-84916 has a columnar convex surface extending toward each nose. Unexamined Japanese utility model publication 64-52613 proposes to form a recess near each nose while unexamined Japanese patent publication 6-179110 discloses an insert having a rake face forming a circular recess as a whole.
In ball end milling, the cutting speed is nearly zero near the nose at the center of rotation. It is high near the terminal end (outer-peripheral side of the tool) of the arc of the arcuate cutting edge extending from the from the nose. The cutting edge is thus subjected to different loads at its different portions. Thus, for optimum cutting edge shape it is required that the cutting edge strength is high near each nose and the cutting resistance is low at the terminal end of the arc of the cutting edge.
The insert of unexamined Japanese utility model publication 1-84916 and those shown in FIGS. 4 and 5 cannot answer this requirement because the rake face has no rake angle and thus the cutting resistance is high. On the other hand, the inserts of unexamined Japanese patent publication 6-179110 and unexamined Japanese utility model publication 64-52613 are too sharp in cutting edge angle at the noses and thus too low in cutting edge strength at the noses.
In the arrangement of the latter publication 64-52613, in which recesses are formed partially in the noses, chips tend to get stuck. In the arrangement of the former publication 6-179110, in which the entire rake face forms a curved recess, the insert is low in strength as a whole and tends to be damaged.
An object of the present invention is to provide a solution to these problems.